1. Field of the Invention
This invention relates to the field of methods of protecting electronic components mounted on a substrate from the ambient atmosphere by providing them with a protective cover which envelopes the components. The perimeter of a segment of a thin flexible membrane which is made of a substantially impermeable nonadhering material is secured by a bead of sealant adhesive at the perimeter of the substrate to enclose the components to be protected. The exposed surface of the segment is coated with a layer of substantially nonpermeable material which is substantially thicker than the membrane of the segment and which does adhere to the membrane. A small opening is formed through the layer and segment. The space under the segment is substantially evacuated, and the opening formed through the layer and segment is sealed.
2. Description of the Prior Art
There is a trend in microelectronic circuit packaging technology toward the use of micropackages comprised of ceramic multilayer printed circuit wiring matrices, or substrates. Each such substrate has formed on it a number of alternate layers of conductive and insulative materials. A large number (i.e., from 50 to 100) of large scale integrated circuit semiconductor chips are mounted within the component area of one surface of a substrate. In a preferred embodiment the top surface of a substrate is substantially a square having dimensions of 80mm by 80mm. To protect the active components, such as large scale integrated circuit chips, hereinafter I.C. chips, their flexible beam leads, and discrete components, such as capacitors mounted within the component area of a substrate from damage by deleterious constituents of the ambient atmosphere, it is the practice to hermetically seal the component space of the substrate. The component space is defined as that space or volume within which both active and passive components of a given substrate are mounted. Hermetically sealing this space has been accomplished by soldering a metallic vertical wall, or riser, to the substrate around the perimeter of the component area and then soldering a metallic cover plate to the top of the riser. The component space is then substantially evacuated and sealed.
The problems encountered in using such a hermetic seal to protect large scale I.C. chips, their leads, discrete components, and the surface of the substrate within the component area from the deleterious constituents of the ambient atmosphere is that hermetic seals fail too frequently and too unpredictably to be used with commercial products such as digital computer systems. One of the reasons for these failures is the consequence of the necessity of soldering different components to a substrate at different times. As a result, it is necessary to use solders which have substantially different melting points. However, the range of temperatures is limited by the maximum temperature to which I.C. chips can be subjected without risk of damage as the maximum and the normal operating temperatures encountered in use of a substrate as the minimum. Since the top plate is soldered last, the lowest temperature solder, which is usually the weakest, is used to solder the top plate to the riser. As a result, it is difficult to consistently obtain solder joints that remain air tight, or hermetic, in use.
Still another probable cause of failures which exacerbates the first is the result of the relatively large forces applied to the top plate and the riser to which the top plate is soldered, particularly because of the large area of the top plate which approaches 56 sq. cms. Thus, any change in ambient atmospheric pressure changes the force on the top plate. Since the atmospheric pressure is constantly changing, so are the forces applied to the seal between the plate and the riser which accelerates failures.
Another problem with hermetically sealed micropackages occurs when any of the active or passive components mounted on the component surface of such a substrate have to be repaired or replaced. To remove the top plate it is necessary to subject a substrate to the amount of heat necessary to unsolder the top plate. Such heat can damage or shorten the useful lives of the remaining components and if not carefully controlled, can result in unsoldering other soldered parts of the micropackage.
A most deleterious component of the ambient atmosphere is water vapor. The reason water vapor is harmful is because it reacts to form acids with chlorides in the residue of the flux used in soldering chips and their leads to the substrate. The resulting acids chemically attack the leads and the I.C. chips which ultimately causes electrical failures primarily where the leads are soldered or bonded either to an I.C. chip or to the substrate.
As a result of this invention, it has been found that it is not necessary to protect active and passive components of a micropackage from changes in ambient atmospheric pressure to adequately protect the chips, their leads and other components mounted in the component area of a substrate from being damaged by constituents in the ambient atmosphere. It has also been found that such components can be adequately protected from the deleterious constituents in their ambient atmosphere by a process which provides a relatively flexible impermeable protective cover for such components which process does not require the micropackage to be subjected to temperatures other than ambient.